Multiple compressor control system

ABSTRACT

At least one of a plurality of compressors connected to a fluid distribution system are loaded and/or unloaded based upon a capacitance of the fluid distribution system.

FIELD OF THE INVENTION

The present invention relates to compression, distribution and control of compressible fluids to optimize supply and demand efficiently without affecting the associated process integrity.

BACKGROUND

Pressurized compressible fluids, such as atmospheric air, carbon dioxide, helium, argon, nitrogen, freon, liquids, etc., are commonly used to deliver energy in the form of pressure in a variety of industrial applications. The devices that use the pressurized fluid, known as load devices, include robots, paint applicators, turbines, power generators, jet engines, pneumatic tools, chillers, air Conditioners and others. Compressible fluids are typically pressurized using a compressor, which may take one of many forms, such as a centrifugal compressor, a reciprocating compressor, a rotary screw, a stack of alternating rotors and stators, or other forms.

A compressor takes in a compressible fluid at an inlet, uses energy to compress a mass of the compressible fluid to a smaller volume and higher pressure, then discharges the fluid thus compressed through an outlet. An individual compressor produces compressed fluid at a specified flow capacity, defined in terms of volume of free fluid at the inlet of the compressor per amount of time. The individual compressor also produces a selected discharge pressure at the outlet due to the normal operation of the compressor. The selected discharge pressure can typically be varied up to a specified maximum discharge pressure of which the compressor is capable.

The specified flow capacity and selected discharge pressure are chosen to suit the particular application for which the compressor is intended. For example, some typical compressors intended for a chemical manufacturing & packaging facility have selected discharge pressures in the general range of 90 to 125 pounds per square inch gage (PSIG), and a flow capacity in the range of 2,000 to 4,000 standard cubic feet per minute (SCFM). SCFM is defined as, “cubic feet of volume per minute at the standard conditions of 14.7 pounds per square inch absolute (psiA) and 60 degrees Fahrenheit.” Many other ranges of discharge pressures and flow capacity are possible depending on the needs of the particular application.

Each load device in turn has a demand flow rate, which is the volume rate of fluid used by the load device in its operation. Each load device also has a specified incoming pressure that it requires for normal operation. Demand flow rate may be fairly constant or change frequently, depending on the application. Any load device is likely to drop its demand flow, rate temporarily at least occasionally for interruptions such as maintenance, breaks, etc.

For facilities in which many load devices are operating, it is common to provide the required pressurized fluid to the load devices through a single fluid distribution system, which services the load devices at its downstream outlets. The single distribution system can in turn be serviced by any number of compressors that supply pressurized fluid to the distribution system at the system's upstream inlets. This single distribution system provides greater flexibility than if each load device had to be serviced by its own compressor, acting to average-out any changes in demand flow rate.

However, total demand flow rate of a collection of load devices still tends to fluctuate during operation. The degree of fluctuation depends on the type and operational nature of the facility using the load devices. If too few compressors are operated, when the demand flow rate rises particularly high, it will surpass the flow rate from the compressors. This will lower the distribution pressure, disrupting the proper operation of the load devices.

To prevent disruptions of this sort, multiple compressor systems are generally designed and installed to cater to the maximum peak demand flow rate at the required load pressure. Facility operators tend to operate the maximum installed capacity of all compressors all the time at the maximum pressure, to ensure that the load devices receive enough pressure even during peaks in demand flow rate. So, the installed compressor discharge flow capacity is greater than it usually needs to be; and the compressors must be set to a higher discharge pressure than what the load devices require most of the time. Excessive compressor capacity and discharge pressure both translate into higher energy consumption, maintenance costs, and capital costs.

However, successful operation of the load devices is typically a greater priority than efficient operation of the compressors. The traditional multi-compressor system therefore sacrifices compressor system efficiency to prevent pressure shortages during times of peak demand flow rate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a multiple-compressor system of the prior art 1.

FIG. 2 is a schematic diagram of a multiple compressor control system according to an example embodiment of the present invention which includes an electronic controller.

FIG. 3 is a Block diagram of one embodiment of the controller of FIG. 1 according to one embodiment

FIG. 4 is a flowchart of an example routine used by the electronic controller of FIG. 3 for controlling the multiple-compressor system shown in FIG. 2, according to one embodiment of the present invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENT

FIG. 1 is a schematic diagram illustrating an example of a multiple-compressor system 100 according to the prior art. In the example shown in FIG. 1, multiple-compressor system 100 is a compressed air system within a chemical manufacturing facility, where the fluid that is compressed is atmospheric air. The facility uses compressed air as energy for cycling valves, cleaning dust collector bag houses, instruments, packaging equipments, conveyors, robots, pulverizing operation, pneumatic tools etc.

Multiple-compressor system 100 includes a plurality of individual compressors C1-C7, which are coupled to a main fluid distribution header 102 through dryer and filter devices D1-D7, respectively. Compressors C1-C7 can be located in one or more areas of the facility. Compressors C1-C7 take atmospheric air in through the respective inlets, compress the air to a higher pressure and discharge the compressed air through the respective outlets. The energy for increasing the pressure of the fluid medium can be derived from one or more prime movers, which drive a shaft of each respective compressor. Each compressor has a specified flow capacity and a specified maximum discharge pressure. The discharge pressures of compressors C1-C7 are typically adjustable within some range up to the specified maximum discharge pressure. FIG. 1 shows an example of the discharge pressure settings for compressors C1-C7. For example, compressor C5 is set to compress the fluid air to a discharge pressure of 115 pounds per square inch gage (PSIG). Gage pressure is the amount by which the total absolute pressure exceeds the ambient atmospheric pressure. All compressors are set to run in upper range modulation control without blow down.

The outlets of compressors C1-C7 are coupled to the inlets of the respective dryer and filter devices D1-D7, respectively. Dryer and filter devices D1-D7 remove moisture, dust and other contaminating particles from the compressed air such that dry, clean air is delivered to main distribution header 102.

Main distribution header 102 is interconnected by welding or other suitable means of fastening, with or without functioning or non-functioning isolating valves. For example, main distribution header 102 can include a combination of 1-inch to 14-inch diameter pipe.

Load devices, such as L1 and L2, can be coupled to outlets along main distribution header 102. As mentioned above, load devices L1 and L2 can include cycling valves, cleaning dust collector bag houses, instruments, packaging equipments, conveyors, robots, for example. Load devices L1 and L2 each have a demand flow rate, which is a volume rate of fluid (air in this embodiment) used by the load device during its operation. Load devices L1 and L2 typically also have a preferred incoming pressure that is required for normal operation. In the example shown in FIG. 1, load devices L1 and L2 require a minimum of 90 PSIG pressure.

The total demand flow rate on main distribution header 102 may be fairly constant or may change frequently, depending on the needs of system 100. Therefore, multiple-compressor systems of the prior art such as that shown in FIG. 1 are generally designed and installed to cater to the maximum peak demand flow rate at the required pressure. If enough of compressors C1-C7 are not running when demand by load devices L1 and L2 increases, the outflow from the system will exceed the inflow to the system causing the density of air in the system and the resulting air pressure to decrease. The decrease in pressure can then cause a disruption in production within the facility. Excessive pressure drops in the system can also be caused by undersized cleaning equipment and piping and dirt accumulated in the system, for example.

In order to avoid pressure drops during periods of fluctuating demand, facility operators tend to operate multiple-compressor systems such that all compressors in the system provide the maximum installed flow capacity and the maximum discharge pressure all of the time. With this type of operation, the average demand flow rate is always less than the installed discharge flow capacity. Therefore, compressors C1-C7 are forced to run at “partial loads”. Partial load is defined by the demand flow rate (SCFM) divided by the discharge flow capacity (SCFM). A compressor is under a partial load when the compressor is capable of supplying a higher flow rate, at the selected discharge pressure, than the demand flow rate.

At partial loads, efficiency of system 100 decreases. Efficiency can be defined as “average SCFM of compressed air/average kW consumed,” where SCFM is the cubic feet of air volume per minute at the inlet of each compressor and kW is the rate of energy consumed, in kilowatts, by the prime mover of the compressor. Efficiency of the total system can then be defined in terms of “total average SCFM of compressed air/total average kW consumed” in system 100.

As a general rule, for every two PSIG increase in discharge pressure of any positive displacement compressor, the energy consumption will increase by one percentage point. Similarly, for every two PSIG decrease in discharge pressure of any positive displacement compressor, the energy consumption will decrease by one percentage point. Therefore a compressor running at 10 PSIG greater than the required pressure consumes approximately 5% more energy than necessary.

Table 1 provides a list of hypothetical properties for compressors C1-C7 according to an example in which system 100 uses air for 8,400 hours per year and maintains around 90 PSIG in the main distribution header. These properties include for each compressor the type, model and make, the designed maximum discharge pressure, the flow capacity (SCFM), the rated energy consumed by the prime mover (kW), the maximum efficiency (SCFM/kW), and a hypothetical measured SCFM, kW and SCFM/kW. TABLE 1 Specifications for Sample Multi-Compressor System: Maximum Discharge Specified Flow Power Compsr. ID: Type: Model: Make: Pressure: Capacity: SCFM Consumed: C1 Rotary W A Corp. 125 PSIG 1,500 SCFM   300 kW C2 Rotary W A Corp. 125 PSIG 900 SCFM 185 kW C3 Rotary Y B Corp. 125 PSIG 900 SCFM 190 kW C4 Rotary Y B Corp. 125 PSIG 650 SCFM 135 kW C5 Rotary Z C Corp. 115 PSIG 1,500 SCFM   285 kW C6 Rotary W A Corp. 125 PSIG 550 SCFM 110 kW C7 Rotary Z C Corp. 115 PSIG 450 SCFM  90 kW Total: 6,450 SCFM   1,300 kW   Specifications for System 100 in operation: Selected Potential Actual CCompsr. Discharge Actual Power Efficiency Efficiency ID: Pressure: Actual Flow: Consumption: (SCFM/kW): (SCFM/kW): C1 125 PSIG 900 SCFM 270 kW 5.00 3.33 C2 125 PSIG 360 SCFM 148 kW 4.90 2.43 C3 125 PSIG 540 SCFM 171 kW 4.7 3.16 C4 125 PSIG  0 SCFM  0 kW 4.80 n/a C5 115 PSIG 750 SCFM 243 kW 5.3 3.08 C6 125 PSIG 220 SCFM  88 kW 5.0 2.5  C7 115 PSIG 180 SCFM  72 kW 5.0 2.5  Total: 2,990 SCFM   992 kW 4.96 3.01

As illustrated in Table 1, all compressors perform at partial load and therefore at a lower than maximum efficiency. Compressor C4 is shown in standby mode. One of the primary causes for the lower efficiency is that the supply rate is more than the demand rate.

Table 2 summarizes the system efficiency of multiple-compressor system 100, shown in FIG. 1. TABLE 2 SYSTEM 100 EFFICIENCY Installed Flow Capacity and Pressure: 6,450 SCFM @ 115-125 PSIG Design Power Consumption: 1,300 kW Design Flow/Power Efficiency Ratio: 4.96 SCFM/kW Average Demand Flow Rate and Pressure: 2,990 SCFM @ 90-100 PSIG Actual Power Consumption: 992 kW Actual Flow/Power Efficiency Ratio: 3.01 SCFM/kW

The compressors in system 100 are partially loaded at an average of 51.5 percent of their flow capacity and have an average total efficiency of only 3.01 SCFM/kW.

FIG. 2 schematically illustrates multi-compressor control system 300 according to one exemplary embodiment. System 300 generally includes a plurality of compressors C1-C7, a plurality of respective drying and filter devices D1-D7, a main distribution header 302, and one or more load devices L1 and L2.

Compressors C1 through 7 are coupled to main fluid distribution header 302 through optional dryer and filter devices D1-D7, respectively. Compressors C1-C7 can be located in one or more areas of the facility, and any number of compressors can be used. Compressors C1-C7 can include any combination of types, makes or models of compressors. For example, compressors C1-C7 can include reciprocating, rotary screw, centrifugal, scroll and vane type compressors. Each compressor has a specified flow capacity and a specified maximum discharge pressure. The discharge pressures of compressors C1-C7 are adjustable within some range up to the specified maximum discharge pressure. In an alternative embodiment, one or more of the compressors C1-C7 have a fixed discharge pressure, and that discharge pressure is selected for the particular application in which the compressor is used. The prime movers for compressors C1-C7 can be driven by electricity, fossil or other fuels, or steam, for example.

The outlets of compressors C1-C7 are coupled to the inlets of dryer and filter devices D1-D7, respectively. Dryer and filter devices D1-D7 remove moisture, dust and other impurities from the compressed air such that dry, clean air is delivered to main distribution header 302. In an alternative embodiment, one or more of the devices D1-D7 can be located in other positions in system 300, such as on the outlet side of its respective compressor. Also, one device D1-D7 can be used to dry and filter air from more than one compressor.

Main distribution header 302 may include a pipe or a series of pipes or other functionally analogous fluid conductors that are capable of conveying pressurized fluid to at least one outlet, such as to load devices L1 and L2. The fluid conductors can be interconnected by welding or other suitable means of fastening, with or without functioning or non-functioning isolating valves. In one embodiment, main distribution header 302 includes a combination of 1-inch to 12 or inch diameter pipe. Other sizes of pipes can also be used.

Load devices L1 and L2 can include cycling valves, cleaning dust collector bag houses, instruments, packaging equipments, conveyors, robots, for example. Load devices L1 and L2 each have a demand flow rate, which is a volume rate of fluid (air in this embodiment) used by the load device during its operation. Load devices L1 and L2 also have a preferred incoming pressure that is desired for normal operation.

In the example shown in FIG. 2, load devices L1 and L2 require about 90 PSIG in main distribution header 302 for normal operation. Compressors C1-C7 are coupled to main distribution header 302. Pre-Engineered Air Receiver 303 is connected to the main distribution header 302. A properly calibrated temperature transmitter 304 and a pressure transmitter 305 are connected to the distribution header 302 preferably at the receiver 303.

As described in more detail below, an electronic controller 306 is connected to the new compressor system 300 through control cables to compressors C1 through C7, Temperature and Pressure Transmitters 304 & 305 respectively. In other embodiments, controller 306 may communicate with compressor C1 through C7 by other means such as optical cables, such as through local area networks or wirelessly.

Electronic controller 306 can be designed to control any number of compressors as long as they are connected to the compressed air system 300 in this embodiment. Electronic controller 306 can be configured to control system 300 in a closed-loop control fashion or an open-loop control fashion. One or more sensors (not shown) can be distributed throughout system 300 as desired for providing electronic controller 306 with appropriate measurements from various locations within the systems. For example, these sensors can include pressure sensors, temperature sensors and mass flow sensors.

Electronic controller 306 comprises a processing unit configured to generate control signals for the direction of one or more of compressors C1-C7. In one embodiment, such control signals are generated based upon capacitance of the fluid distribution system. For purposes of this disclosure, the term “processing unit” shall mean a conventionally known or future developed processing unit that executes sequences of instructions contained in a memory. Execution of the sequences of instructions causes the processing unit to perform steps such as generating control signals. The instructions may be loaded in a computer or processor readable medium 307 that may comprise a random access memory (RAM) for execution by the processing unit from a read only memory (ROM), a mass storage device, or some other persistent storage. In one embodiment, memory 307 may be removable and portable with respect to controller 306. In other embodiments, hard wired circuitry may be used in place of or in combination with software instructions to implement the functions described. Controller 306 is not limited to any specific combination of hardware circuitry and software, nor to any particular source for the instructions executed by the processing unit.

Electronic controller 306 can include any control device such as a programmable logic controller (PLC), a microprocessor-based controller, or a personal computer-based controller. Electronic controller 306 can be a digital-based or analog-based controller. In alternative embodiments, electronic controller 306 can be replaced with a plurality of individual controllers, wherein each controller controls one or more of the components within system 300. In addition, electronic controller 306 can be replaced with a manual-type control, a different electrical-type control or a combination of both.

The receiver sizing depends on the air demand variance in SCFM over time as in the prior art, the start up time (T1, T2, etc.) required by compressors C1 through C7 and the set pressure (Ps in PSIG) and the swing tolerance (for e.g. plus 3 PSIG and minus 3 PSIG) permissible by the plant operation. Receiver 303 is designed to have a volume of 6,000 gallons (780 CF). Receivers having other volumes can also be used. The volume of the entire distribution header piping including the volumes of the filters, cleaning equipment is measured as 40 CF (same as the system in FIG. 2) Therefore the total volume of the system 300 is 820 CF.

One of the factors influencing the control algorithm is the useful storage or storage capacitance (C). Storage capacitance is defined as Value of Standard Cubic Feet of Free Air (Air at Standard Conditions) in an air system, to effect a change (plus or minus) in the system pressure by 1 PSI. It is calculated by the formula P1V1=P2V2, where P1 & V1 are the pressure and volume during time 1, and P2 & V2 are the pressure and volume during time 2. Capacitance of any system can be closely approximated to C=V/Pa, where C=is the capacitance of the system in Standard Cubic Feet (SCF)/PSIA, V is the total Volume of the system in Cubic Feet (CF), and Pa is the Ambient Pressure at the standard condition, 14.7 PSIA.

In the example represented by FIG. 2, system 300 requires an average volume rate of 2,990 SCFM of compressed air to be delivered to main distribution header 302 at a minimum of 90 PSIG. Table 4 provides a list of hypothetical specifications for compressors C1-C7 according to the example.

Given the flow capacities of each compressor and their efficiencies, the minimum number of compressors C1, C2 & C4 are selected to run. C1 & C2 are run by the electronic controller at full load (inlet valve fully open) delivering 2,400 SCFM to the distribution header. The balance 590 CFM is delivered by C4 (90% of the cycle time fully loaded and 10% of the time fully unloaded). These compressors are set to provide the minimum discharge pressure that is practically acceptable for the proper operation of the load devices. The pressure requirement of the plant is a minimum of 90 PSIG at the plant main header 302.

The electronic controller 306 is therefore programmed to maintain a minimum header pressure of 90 PSIG at the distribution header 302. Since the plant header pressure is always sensed by pressure transducer 307 after the drying and cleaning equipment D1 through D7 in this embodiment the pressure losses in the drying and cleaning equipment do not affect the efficiency of loading and loading or starting or stopping of compressors C-1 through C-7.

In the example shown in Table 3, compressor C1 has a discharge flow capacity of 1,500 SCFM, and compressor C2 has a discharge flow capacity of 900 SCFM. System 300 therefore requires a balance of 590 SCFM at minimum 90 PSIG and that is supplied by the Compressor C3 whose capacity is 650 SCFM. TABLE 3 Specifications for System 200 in operation: Ave. Design Actual Actual Dis. Flow: Flow: Power DESIGN Actual Comp.# PSIG: SCFM SCFM kW SCFM/kW SCFM/kW C1 98 1,500 1,500 260 5.00 5.8 C2 98 900 900 161 4.90 5.5 C3 98 650 590 109 4.80 5.4 C4 98 550 0 0 4.7 n/a C5 98 1,500 0 0 5.3 n/a C6 98 900 0 0 5.0 n/a C7 98 450 0 0 5.0 n/a Total: 2,990 530 4.96 5.6

Looking at Table 3, since compressors C1, C2 operate at their maximum flow rates, and C4 operates very near to full load capacity, these compressors have larger SCFM/kW efficiencies than similar compressors in the prior system shown in Table 1.

Table 4 summarizes the overall efficiency of system 300, which can be compared to the efficiency of system 100, as shown in Table 3. TABLE 4 SYSTEM 200 EFFICIENCY Active Flow Capacity and Pressure: 3,050 SCFM @ 90-96 PSIG Average Demand Flow Rate and Pressure: 2,990 SCFM @ 90-96 PSIG Compression Flow Demand/Supply Ratio: 98% Average Power Consumption: 530 kW Flow/Power Efficiency Ratio: 5.6 SCFM/kW

The system operating configurations as defined Table 3 & 4 are ideal if practically achievable.

With the available controls from prior arts, the ideal conditions of running optimum number of compressors are not reliably and consistently achievable. The reliability and consistency are not completely achievable in prior arts because the controls do not take the system capacitance in to consideration while loading and unloading the compressors. The plant operator feels confident and secure when the system is consistent and reliable. If the plant operator is not confident and secure, he would typically by pass the controller and energy saving or efficiency project will be a failure.

The control algorithms of controller 306 may eliminate situations as above and automatically calculates and selects the lead time on a dynamic basis, depending upon the changing system parameters at any time.

The control algorithms are developed based on the available capacitance, the change in demand, constantly varying sampling period and the corresponding recovery and lead periods, the permissible minimum pressure and tolerance, each available compressor capacity and the minimum time required to go on-line and load. Also, with system 300 the need for a flow controller and intrusive installation efforts are eliminated without compromising on the system pressure requirements. This greatly reduces installation cost.

As per one embodiment, all running compressors except one floating trim compressor in system 300 are fully loaded. Considering the same example as described above, the demand flow of 2,990 SCFM in system 300 is supplied by the three compressors C1 & C2 at full load and C3 loading at 90 PSIG and unloading at 96 PSIG (93 PSIG is the set pressure with tolerance of plus or minus 3 PSIG). Out of the 650 SCFM compressed by C3, 590 SCFM is consumed by the load devices L1 & L2, the balance of 60 SCFM raises the pressure of the header 302 from 90 to 96 PSIG. The system capacitance is 820 CF/14.7 PSI=55.7 SCF/PSI. If the demand in the system increase by 360 SCFM, when the system pressure is 91 PSIG and the compressor C3 is in its unload cycle, the electronic controller 306 will immediately load the running compressor #3 after allowing for the lead time required by C3 for loading, say 1 second in this case. The total air demand is 3,350 SCFM and the total supply capacity of all the compressors is only 3,050 SCFM which results in a short fall of 300 SCFM or 5 CF/second. The pressure in the system will still be dropping. In this embodiment of the control algorithm calculates the sampling period, and corresponding recovery period and response time (instead of 5 seconds as set in the prior art) before starting and loading compressor C-7 whose capacity is 450 SCFM. The capacitance of the system 300 is 55.7 SCF/PSI. The short fall is 5 CF/sec, the current system pressure is 91 PSIG, the lower pressure set limit is 90 PSIG, and the controller 306 will determine the sampling period and the corresponding maximum response time as ((91−90)*55.7 CF/PSI)/(5 CF/sec)=11.14 seconds before a compressor is to be started and loaded. The controller has determined the short fall as 5 CF/sec. and immediately looks for the compressor whose capacity is the closest to the to 5 CF/sec. from the available compressors. The controller selects C7 whose capacity of 7.5 SCF/sec. is the closest (from Table 3) to the short fall of 5 SCF/sec. from the Table 3. After selecting C7 it compares the response time required by C7 and matches that with the determined sampling period and the maximum response period. If the response period of C7 is less than the maximum response time determined by 306, it sends a signal to start and load C-7 and the appropriate time. For example the start and load response time is 5 seconds, the controller 306 will start the compressor at the 6^(th) second of the available 11.14 seconds. Then, the supply capacity is 3,490 SCFM as against the demand of 3,350 SCFM. The surplus 140 SCFM will raise the pressure of the system 300 from around 90.5 PSIG to 96 PSIG (set pressure of 93 PSIG plus 3 PSIG tolerances) in 131 seconds unless the Air Demand in the system increases further during the pressure rise. Therefore the system will not take any action for 131 seconds (as against the controller per the prior art) and wait till 131 second had almost elapsed before unloading the compressor whose capacity is closer to the surplus flow. In other words it will unload C-7. Now the short fall for the same demand is 5 SCF/sec and the controller will determine the sampling period is at least 66 Seconds by the same methodology as described before, before loading compressor C-7 again. This cycle of compressors C-1 & C-2 fully loaded and cycling (load and unload) compressor C-7 will continue as long the as the demand remains closer to 3,350 SCFM.

As a result, multiple-compressor system 300 is made stable and this situation is conducive for the operator feeling secure about the stability of the system (as against the previous example where three compressors follow a load/unload cycle in a short period of time).

Therefore, projected power consumption savings of approximately 462 kW is practically feasible with controller 306.

In the above example, compressor C-4 was the floating trim compressor for a demand of 2,990 SCFM and compressor C-7 was the floating trim compressor for a demand of 3,350 SCFM. Likewise, for any change in demand either lower or higher, multiple compressor controller will maintain only one floating trim compressor.

FIG. 3 is a block diagram illustrating a control function of electronic control 306 in greater detail. In one embodiment, electronic control 306 includes a programmable logic controller (PLC) 400 having a program 401 and a database 402. Program 401 is tailored to perform the desired control function for the multiple-compressor system based on data stored in database 402 and input parameters received from the pressure sensor 307 and temperature sensor 308, for example. Program 401 can be implemented in software, hardware or a combination of both.

Database 402 includes system-specific data, such as the specifications of each component in the system. These specifications can include the maximum discharge pressure, the selected discharge pressure, the maximum discharge flow capacity and the rated energy consumption of each compressor, the pressure consumption of each drying and filter device, the flow capacity of each dryer and filter device, the total system flow capacity, the “total volume” of system 300, etc.

FIG. 4 is a flowchart illustrating the steps performed by PLC 500 in controlling the various components within multiple-compressor system 300 according to one embodiment.

At step 500 data is provided to the multiple compressor controller from database 402 (shown in FIG. 4) and from the various sensors in the system. At step 501 the system is turned on and initialized. The controller is powered-up and selects the desired compressors to be started and loaded. Step 401 can be performed at the start of each work day in a facility or at less frequent times if the facility operates 24 hours per day.

At step 502, the multiple compressor controller 306 calculates the mass rates of change, the response time to start and load or unload and stop based the capacitance in the system, at which point of the dynamics the action is to be taken, by using the system data from 500.

This calculation is based on inputs to the multi-compressor controller 306 from sensors, transducers, available time, an internal memory storage, a network-hosted database, or other input sources. The inputs represent values for the system pressure, temperature, set pressure, the tolerances, the capacitance, and the starting characteristics of the compressors C1 to C7 available at that time of sampling time and time to reach the upper or lower limits of the set pressure.

In one example, a method of calculating air density is used wherein a standard air density under arbitrarily chosen conditions forms a STANDARD value, which is subjected to correction terms such as temperature and pressure to reach an accurate value for local conditions. A calculation of air mass in the system 300 can therefore take the form of: M _(t)=(D _(s) *V _(t))/[(Pa*(T+460))/((Pa+P _(t))*(T _(s)+460))] where M_(t) is the mass of air in the system 302 (receiver 303 and the piping), D_(s) is a standard air density at standard conditions of temperature and pressure, V is the system total volume, which is the volume of the receivers, the filters, dryers, and the piping etc. T is the measured temperature of the air in degrees Fahrenheit, T_(s) is standard air temperature in degrees Fahrenheit, Pa is the standard ambient pressure in psiA, and Ps is the system pressure in psiG. The term of 460 added to both temperatures sets them to an absolute scale by compensating for absolute zero being 460 degrees below zero in the Fahrenheit scale. Details of the equation would change in other embodiments, such as if temperature were measured in the Kelvin or Celsius scale, or if additional corrective terms were included, according to well-known methods of calculating a mass based on values of pressure, volume, density, etc. In an alternative embodiment, the rate of change in mass is calculated for the receiver only. In this embodiment, V represents the receiver volume.

The rate of change of mass is calculated for the current sampling period (t1) which is dynamically varying as per the system dynamics and is determined by the customized program 502, at intervals of t1 seconds. For example, if t1=30 seconds and t2=1 seconds, the PLC would calculate six samples of the mass rate of change over a 30 second time period at 1 second interval.

If the mass rates of change on the system indicates a downwards slope as indicated in step 503 of the FIG. 4, then the step 504, 505 & 506 determine if the downwards slope is negative, positive or the same respectively. If 504 is determined no action to start and load a compressor is needed and accordingly the loop completes back to 402. If 505 is determined then step 506 determines if the downward slope is due to the floating trim compressor, then step 507 takes “No Action” to start and load another compressor and the control loop goes back to 502. If step 506 determines that the mass rate of change slope is not the same as that of the downward trend of the floating trim compressor, the step 508 is directed to step 509. Step maps the rate of change of mass and determines the load the unloaded floating compressor (if it is in its unload cycle), calculates the response time, and accordingly loads the floating compressor at a comfortable time based on the capacitance and available mass, the load time characteristics of the floating compressor. If the rate of change of mass is still downwards, step 505 takes over and the process of comparing the mass short fall, available sampling and response times and mapping with the available compressor in step 502. Step 510 then determines rate of change upwards and compares with the floating trim compressor and unloads the floating trim compressor when the system pressure is reaches its upper tolerance level.

The process is continuous and the cycle of calculating the sampling period, response time etc. is repeated maintaining the system pressure within the tolerance level and at the same time controlling to sustain only one compressor as the floating trim.

If the slope of the mass rate of change of slope is upwards as determined in step 511, then step 512 compares the slope with that of the floating compressor in its loading cycle and if the calculated slope is same, no action to unload the floating compressor is taken. If the upwards slope is not the same and depending the then current sampling period, time to unload any compressor, it unloads the floating trim compressor. Then step 515 waits for the selected stop delay period for the floating compressor and stops the prime mover (motor or generator). If one particular floating compressor is unloaded, then the multiple compressor controller selects one of the remaining running compressors as the floating compressor and maintains stability in the system and prevents frequent load and unload cycle of the more than one compressor.

Steps are dynamic as the system air flow changes. The particular steps taken by the multiple compressor to maintain pressure within the main distribution header are provided as example only. Numerous modifications can be made in alternative embodiments of the present invention. Further, representations of the mass rate of change can be calculated in a number of ways. For example, the multiple compressor can calculate the rate of change of mass or pressure.

In summary, the multiple-compressor control system of the present invention provides an economically feasible, much less expensive and practical solution to the problem of improving operating efficiency of the system as indicated by the “total average compressed SCFM/total average kW consumed.” This translates to reduction in the energy consumed by the system, the cost of components used in the system, maintenance expenses and other ancillary costs. The system also provides a stable pressure within a close tolerance to the desired pressure in the plant header. A stable pressure reduces production disruption and increases productivity.

Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. 

1. An apparatus comprising: a controller configured to selectively perform at least one of loading and unloading of at least one of a plurality of compressors to a fluid distribution system based upon a capacitance of the fluid distribution system.
 2. The apparatus of claim 1, wherein the controller is configured to selectively perform at least one of loading and unloading based upon a mass rate change of fluid in the fluid distribution system.
 3. The apparatus of claim 2, wherein the controller is configured to selectively perform at least one of loading and unloading based upon a lead time for the at least one of loading and unloading the at least one of the plurality of compressors.
 4. The apparatus of claim 1, wherein the controller is configured to selectively load at least one of the plurality of compressors based upon a capacitance of the fluid distribution system.
 5. The apparatus of claim 4, wherein the controller is configured to selectively load the at least one of the plurality of compressors based upon a minimum fluid distribution system pressure.
 6. The apparatus of claim 4, wherein the controller is configured to selectively unload the at least one of the plurality of compressors based upon a capacitance of the fluid distribution system.
 7. The apparatus of claim 6, wherein the controller is configured to selectively load and unload the at least one of the plurality of compressors based upon a range of acceptable fluid distribution system pressures.
 8. The apparatus of claim 1, wherein the controller is configured to selectively unload the at least one of the plurality of compressors based upon the capacitance of the fluid distribution system.
 9. The apparatus of claim 1, wherein the controller is configured to select the at least one of the plurality of compressors from the plurality of compressors for at least one of loading and unloading based upon a capacity of each of the plurality of compressors.
 10. The apparatus of claim 1, wherein the controller is configured to adjust a sampling period for sampling a mass rate of change in the fluid distribution system based upon a capacitance of the fluid distribution system.
 11. The apparatus of claim 1 further comprising: a fluid distribution system; and a plurality of compressors, at least one of the plurality of compressors being configured to be selectively pneumatically connected and disconnected from the fluid distribution system in response to control signals from the controller.
 12. A processor readable medium comprising: stored instructions to selectively perform at least one of loading and unloading of at least one of a plurality of compressors connected to a fluid distribution system based upon a capacitance of the fluid distribution system.
 13. The processor readable medium of claim 12 further comprising stored instructions to determine a floating mass rate change of fluid in the fluid distribution system and to selectively perform at least one of loading and unloading of at least one of the plurality of compressors connected to the fluid distribution system based upon the floating mass rate change.
 14. The processor readable medium of claim 11, wherein the stored instructions to selectively perform at least one of loading and unloading is based upon a lead time for starting and loading the at least one of the plurality of compressors.
 15. The processor readable medium of claim 1 1 further comprising stored instructions to selectively perform loading and unloading of at least one of the plurality of compressors additionally based upon a range of acceptable fluid distribution system pressures.
 16. A method comprising: selectively performing at least one of loading and unloading of at least one of a plurality of compressors connected to a fluid distribution system based upon a capacitance of the fluid distribution system.
 17. The method of claim 16, wherein at least one of loading and unloading of at least one of the plurality of compressors is based upon a mass rate of change of fluid in the fluid distribution system.
 18. The method of claim 16, wherein the at least one of loading and unloading of at least one of the plurality of compressors is based upon a lead time for starting and loading the at least one of the plurality of compressors.
 19. The method of claim 16, wherein the at least one of loading and unloading comprises selectively loading and unloading of at least one of the plurality of compressors based upon a range of acceptable fluid distribution system pressures.
 20. The method of claim 16 further comprising adjusting a sampling period based upon a mass rate of change of fluid in the fluid distribution system. 